With the increase in density and accuracy of ultra-LSI devices in recent years, the requirement to flatness and surface defect of mask blank glass substrates is becoming stricter year by year. As a conventional precision polishing method for reducing the surface roughness of a mask blank glass substrate, there is one disclosed in JP-A-564-40267 (Patent Document 1), wherein surfaces of a mask blank glass substrate are polished using a polishing agent mainly composed of cerium oxide and then are finish-polished using colloidal silica.
Then, the polished glass substrate is cleaned, thereby removing foreign matter such as the polishing agent adhering/remaining on the substrate surfaces. As a conventional substrate cleaning method, there is one disclosed in JP-B-3879828 (Patent Document 2), wherein a substrate is treated with a low-concentration hydrofluoric acid aqueous solution, then cleaned with an alkaline solution, and finally washed with water (rinsed).
The mask blank glass substrate is manufactured in this manner and then a light shielding film or a phase shift film and so on are formed on the upper surface of the obtained glass substrate, thereby obtaining a photomask blank.
In recent years, following the demand for further miniaturization of semiconductor devices, the extreme ultraviolet (EUV) lithography being an exposure technique using EUV light has been expected to be promising. The EUV light represents light in a wavelength band of the soft X-ray region or the vacuum ultraviolet region and, specifically, light having a wavelength of about 0.2 to 100 nm. As a mask for use in the EUV lithography, there is proposed an exposure reflective mask described in JP-A-2002-122981 (Patent Document 3).
The proposed reflective mask has a multilayer reflective film formed on a substrate and adapted to reflect exposure light and further has an absorbent film formed in a pattern on the multilayer reflective film and adapted to absorb the exposure light. The exposure light incident on the reflective mask disposed in an exposure apparatus (pattern transfer apparatus) is absorbed at a portion where the absorbent film is present, while, is reflected by the multilayer reflective film at a portion where the absorbent film is not present so that a reflected optical image is transferred onto a semiconductor substrate through a reflective optical system.
In the case of such an EUV reflective mask blank substrate, the requirement to its surface defect is extremely strict. That is, when a reflective mask blank and a reflective mask are fabricated using a glass substrate having on its surface a convex defect due to adhesion of foreign matter or the like, if the convex defect exists near a mask pattern, reflected light of exposure light is subjected to a change in phase due to the convex defect. This change in phase causes degradation of position accuracy or contrast of a transferred pattern. Particularly when short-wavelength light such as EUV light is used as exposure light, the change in phase becomes very sensitive to minute irregularities on a surface of the mask and thus the influence thereof on a transferred image increases, and therefore, the change in phase caused by the minute irregularities is an unignorable problem. In fact, when EUV light having a wavelength of, for example, about 13 nm is used as exposure light, even a convex defect of about 2 nm can cause a phase defect. In the case of the EUV reflective mask blank, since a film formed by, for example, alternately laminating Mo and Si layers, each having a thickness of several nm, by about 40 to 60 cycles is used as the above-mentioned multilayer reflective film on the substrate, even if there is a minute convex defect of a degree that causes no particular problem on a surface of the substrate, it may happen that when the multilayer reflective film is formed, the size of the defect on the substrate surface increases to generate a convex defect large enough to cause a phase defect on a surface of the multilayer reflective film.
For the reason described above, particularly in the case of the EUV reflective mask blank substrate, it is necessary to satisfy the condition of very high level for the surface defect.